This invention relates to a printing apparatus. The invention has particular application to high speed printers wherein impression dies are positioned adjacent paper for the formation of impressions on the paper. In a typical situation, a ribbon is employed whereby a visible impression is achieved.
In order to provide maximum operating speed, reciprocating hammers are employed as the means for supplying the force to achieve a suitable impression. The hammer may be supported on a carriage along with a ribbon, and the carriage is driven adjacent a platen or the like which supports a sheet of paper. In a system such as described in Martin, et al. U.S. Pat. No. 4,030,591, the impression dies are positioned at the end of spokes of a rotatable printing disc. This disc is also supported on the carriage, and the operating sequence involves shifting of the carriage from one position to the next and simultaneous shifting of the disc to locate the next desired die opposite the hammer. The hammer is then driven into engagement with the spoke supporting the die whereby the die will engage the adjacent ribbon which, in turn, forms the impression on the paper. The sequence is then repeated wherein the carriage and disc shift to the next position, and the hammer again operates.
In one alternative arrangement such as described in Goin U.S. Pat. No. 3,072,045, the impression dies are formed on a wheel, and paper and ribbon are moved adjacent the wheel. The hammer is located on the side of the paper opposite the wheel.
Although very high operating speeds can be achieved with systems of the type referred to, certain problems are recognized. Specifically, the necessity for continuously driving the hammer at high speed and with suitable force creates conditions leading to significant wear. The life of parts can thus be relatively short necessitating undesirably frequent equipment maintenance.
The wear problem also particularly effects the quality of printing achieved. In order to provide suitable quality, it is necessary that the hammer strike at a precise position since otherwise the printed character will not have a consistent appearance. For example, in the case of a printer utilizing a rotating disc, it is necessary that the hammer engage the spokes with precision in order to provide uniform engagement of the dies with the ribbon and paper. If the hammer strikes a spoke at an angle, the spoke and the associated die would be tilted so that one portion of the die would engage the ribbon and paper with different force than another portion thereby resulting in uneven character appearance.
It has also been recognized, for example as described in Helms U.S. Pat. No. 3,172,353, that the energy imparted to a hammer die significantly affects the appearance of printed material. Specifically, if a single energy level is imparted, the surface area of the character involved will determine the appearance of the printed character. A "." or a "," thus has a significantly smaller surface area than an "M" or "W". The same force applied by the hammer would thus result in a lighter print for the "M" or "W" if a low energy level is involved. On the other hand, if a high energy level is involved in order to achieve sufficient force for the "M" or "W", then there is a significant possibility that the "." or "," die will penetrate the paper.
Different energy levels have been proposed, for example in structures utilizing solenoid driven hammers. Thus, two or more levels of power are made available with the particular connections to the solenoid depending upon the particular character to be printed. The aforementioned Goin patent also addresses this problem by utilizing a coil movable in a magnetic field. The energy imparted to a hammer connected to the coil is controlled by varying the current supplied to the coil.
Helms U.S. Pat. Nos. 3,172,352 and 3,172,353 also discuss the use of coils attached to a hammer and situated within a magnetic field to provide driving force. In these cases, the hammer is mounted on flexible rod-like supports while in Goin, the hammer is attached to a diaphragm or deformable disc.
Although addressing the problem to a degree, the prior at has not developed a completely suitable combination which avoids the aforementioned wear and alignment problems while also permitting significant selectivity from the standpoint of energy imparted to a reciprocating hammer.